When a gyrostabiliser is used in a marine vessel to attenuate rolling motion, the rolling motion of the vessel induces flywheel precession that in turn produces a torque that opposes the rolling motion. This means that the induced flywheel precession is always in an appropriate phase to attenuate rolling motion (if the precession does not exceed 90°). Depending on the resistance to precession caused by the mechanical arrangement, when the rolling rate of rotation (roll rate) exceeds a certain level, the induced precession torque will cause the flywheel to precess through more than 90° (over-precess). This causes instability in the roll resisting torque produced by the flywheel as it momentarily becomes zero. If the resistance to precession is great enough to prevent induced over-precession during peak input events, the stabilising effect during more common events will be severely limited. It is therefore desirable to provide a gyrostabiliser control system to either vary the resistance to precession, or actively control the flywheel precession motion of the gyroscope.
There are many known gyrostabiliser control systems, from the manual precession axis brake actuated by a lever to active control of the precession angle in dependence typically on sensed vessel roll motion and gyroscope precession motion. The manual precession axis brake, first proposed by Schlick in 1904 and described by White in 1907, required manual intervention to prevent over-precession in wave environments outside a small range of design conditions.
The American company Sperry then developed a system that addressed the problem of the Schlick gyroscope by using an electric motor controlled by switches and a small gyroscope to control the precession of the main gyroscope. In this system the rate of precession was proportional to the roll rate of the vessel. Although the performance of these, prior art systems was remarkable in some wave environments, (up to 95% roll reduction), the precession control systems were not able to adapt to varying wave conditions, so the performance was limited by simplistic precession torque controls. With the invention of lighter and cheaper fin stabilisers, which work well when a ship is at speed allowing hydrodynamic lift to be produced by the fins, interest in gyrostabilisers waned.
Gyrostabilisers have particular benefits for applications where the vessel has zero or low forward speed, when hydrodynamics based systems have little or no effect. Several applications including, but not limited to, patrol boats, luxury motor yachts, offshore floating production systems and offshore work boats all have significant operational roles at low or zero speed. These applications are driving renewed interest in revisiting gyrostabilisers for controlling wave induced ship rolling motion.
As a result, there have been proposed more complex control systems for gyrostabilisers to provide improved vessel motion attenuation performance over a wide range of operating conditions. For example, in WO 2009/009074, Rubenstein and Akers disclose a control strategy using attitude and angular rate sensors for both the vessel and the gyrostabiliser to produce a feed forward component. This is used along with a feedback component, a mode input (indicative of current events such as launching, parked or underway at various speeds) and an anticipation of the effect of the intended control (when applied to the gyrostabiliser and/or other vessel stabilising devices) to produce a resource allocation vector for the gyro and any other control means.
By actively controlling the precession of the gyrostabiliser flywheel, safe and effective performance across a wide range of operating conditions can be achieved. Active control of the precession requires sensor feedback for use as process control variable(s). If a process control variable becomes unavailable to the control system due to sensor error, loss of system power or other failure, the active control system will cease to operate. For actively driven gyrostabilisers, this will result in immediate loss of the stabilising influence of the gyrostabiliser, which risks the safe and/or comfortable operation of the vessel.
With the provision of increasing numbers of sensors to permit more complex gyrostabiliser control algorithms that are more responsive to changing sea states and vessel motion, the risk of one or more sensors failing is increased.
The present invention was developed with a view to providing a gyrostabiliser control system and method that is fault tolerant, reducing the severity of the risk of one or more sensors failing. However it will be appreciated that the control system and method of the invention also has application to other types of vessel stabilisers, such as hydrodynamic stabilisers. Examples of hydrodynamic stabilisers include, fins, rudders, T-foils, interceptors, flume tanks or transom flaps and examples of other types of stabilising device include moveable ballast systems.
References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.